Coded track circuit signaling



CODED TRACK CIRCUIT SIGNALING SYSTEM Filed Nov. 9, 1940 3 Sheets-Sheet lINVENTOR March 13, 1945. N. D. PRESTON CODED TRACK CIRCUIT SIGNALINGSYSTEM I 3 Sheets-Sheet 2 Filed Nov. 9 1940 INVENTOR WMM March 13, 1945.PRESTON 2,371,263

CODED TRACK CIRCUIT SIGNALING SYSTEM Filed Nov. 9, 1940 3 Shee ts-Sheet5 FIG-.3. F G-4- mvEiifbR Patented Mar. 13, 1945 SYSTEM Neil D. Preston,Rochester, N. Y., assignor to General Railway SignalCompany, Rochester,

Application November 9, 1940, Serial No. 365,065

14 Claims.

Thisinvention relates to so-called coded track circuits for railroads,and more particularly to the provisionof means for protecting the trackrelay operated by the inverse or off-code pulses from being improperlycoded if a stray or extraneous potential across the track railsshouldexist a1; a time when the inverse pulses are cut oif and this trackrelay should not operate.

-In the conventional coded track circuit, to which this inventionrelates, a battery or other source of current at one end of a tracksection is intermittently applied across the track rails to energizeanddeenergize a code following track relay at the other end of the sectionin accordancewith a suitable code pattern, which is usually the rate ofthe intermittent energization. The character or rate of this transmittedor driven code may be changed to conform with traffic conditions, of thelike, and thus provide for the trans-- mission over the track rails ofdifferent condi-' tions of control for governing the indication ofsignals, or thelike, as well as detecting the occupancy of the tracksection. In certain applications of such coded track circuits, it isdesirable to provide another track relay at the feed or driv- I .at-theexit end of a track section may also respond to theoccupancy of thesection, orother control of'the inverse or off-code pulses. Inotherwords; in this type of coded track circuit a driven code of a selectedcode character is transmitted in one direction over the track section togovern decoding means at one end, and an inverse code is transmitted in;the opposite direction during the off intervals of the driven code togovern decoding means at the other end of said section. vIn the usualarrangement for a coded track circuit of this character, the track relayat the .feed orexit end of the track section and normally responding tothe off-code pulses, is intermittently connected across the track rails;and it can be readily seen that, if some extraneous stray "potentialshould exist across thetrack rails of i the proper polarity andintensity to energize this H oil-code relay, it will be automaticallyenergized and deenergized intermittently as it is connected to anddisconnected from th'isstray potential by operation of the codetransmitter, and will not stop its coding operation as it should whenthe oil-code pulses cease. In other words, the code transmitter for thedriven code may act in the presence of a stray current to cause a falseselfare removed.

coding action of the off-codetrack relay intended to respond to only theoiT-code pulses transmitted from the other end of the track circuit, andthereby create an unsafe condition if the inverse code is relied uponfor safety.

This false self-coding of the off-code track relay by a stray current isparticularly objectionable where the inverse code is cut 01? before atrain enters the section for thepurpose of relaying some desired controlover a plurality of track circuits. When a train entersa coded trackcircuit section at the end receiving the transmitted or driven code, itof course tends to shunt the oiT-code track relay at the other end; butt may happen that the linear impedance of the rails, particularly ifthere is high resistance bonding, together with a suflicient inter-railstray potential maintained in spite of the Wheel shunt, will allowsufiicient current to flow through the 'ofi" code relay to cause itsimproper operation.

The primary object of the present invention is to provide a coded trackcircuit organization affording an inverse or off-code which is automatiecally checked or protected against false coding operation in thepresence of a stray current, so that the inverse code may be relied uponsafely to govern approach locking relays, indications of opposingsignals, or the like.

Generallyspeaking, and Without attempting to define the exact nature andscope of the invention,,it is proposed to protect against such impropercoding of the oii-code track relay in the presence of stray current byrequiring this relay, in order that its operationwill be efiective toindicate the presence of the inverse code, to release positively duringeach ofi'period of the transmitted code, such release occurring in thecase of the legitimate inverse code pulses, but not in the presence of astray current capable of falsely energizing said relay when the inversecode pulses In one embodiment of the invention, whenever the off-codetrack relay fails to release during the ofi period of the driven code,the operation of the transmitter relay or equivalent meansintermittently connecting this relay to the track rails is automaticallystopped, so that there can be no self-coding of this relay. In thisembodiment of the invention, a stray current capable of causing improperself-coding of. the oil-code track relay acts to stop coding in bothdirections, putting to'stop the signal governed by the driven code andcreating a condition showing that a stray current exists.

In another embodiment of the invention, the

and attributes of the invention will.appear.as-.-the

description progresses. i

The accompanying drawings illustrate in a simplified and diagrammaticmanner-various specific embodiments of the inventiony thenature of theillustration being designed to facilitate an understanding of theprinciples; and mode {OI/1.101381%- tion of the invention rather than toshow the exact arrangement and construction ofthe parts .that ight beemployed iinpractice. .Imthese drawings, Fig. .1 illustrates oneembodiment .of

they invention in .ajsimplefform. applicabl to various conditions. and.systems of .tralfficcontrol .for railroads; 'Fig. 2 illustrates .the-organizat-ionof Fig..1 usedfor-approach locking controhaandy Figs. 3and 4.show various.variationsand.modifications .in thespecificarrangementof partsforreceiving the off-.codepulses.

.Referring first .to the-embodiment of thetinvention I'iIIustrated-inFi1, it .is.-assumed :that the codedtrack. circuitis. of. thconventionaldirect current .type, .using primary -batteries ortrickle-charge storage batteries as a source TfOl' the track circuit.current, but .it should be understood that the same .principles .may beapplied to'. coded track circuits :energized from: .alternating currentsources. It :is -:also contemplated that -transmittedior .driven codesofdifferent rates suchl 8,57513111-(1 180. on and 'ofi periods perminute; willbe obtained from aesuitableicode generating tdevice, ":SllChI as .a code v. oscillator, :in accordance with\ the usual practice, thecontacts of such. an oscillator being illustrated diagrammatically. asC180 and-.C'l:5. -iFortthe purpose :ofr .thisdisclosure,-.contactsflltln and .G15iareuassumed to be on separateoscillators which are normally operatingiatrtheirlrespective'rates. )Theselection between: the different ::code 1 rates '2 to be eiTective atany :giVGnr time :for :deterniiningrzthe indication zof 'the .:signalcor other (means governedvby the -.rlriven :code may the raccomplishedin any suitableimannersatndzthe rrelayCXis typical or illustrativerofaarmeans isuitable ifor this'murpose.

,In the particular arrangement illustrated-in Fig. 1,it..is.assumed:that a'tnansmitted or driven code from f rightzto left,was indicated by the solid arrow, .willibe employed for (governingindications of a waysidesignal Lland thatam inverseorofi code in theother direction, as indicated by the dotted arrow,"=w-ill :be employedto control the energizationbf a? relay OOHpontrolling'the indications ofsomes ot'her signal, the approach looking equipment 1 for :a switch, :orfor some other I purpose. It :is further contemplated-that the 'in verseor ofi'...code:may be:'controlled 'by the-presence of 1 a train inioneor more adjoining "track sections, or in *ssome bother manner,'dependent upon? the use .of :the; coded :track: circuit; and the relayYinllig. 1a isitypical or;iill11strative:of: 'such :a meansfor cutting:fofinthesinverse Lcode Lprior to and independently of theentranceaofzaltrain into the section.

In the arrangement =-illustrated, othercoder on" oscillator contacts Cl80 or C as selected by relay X energize a repeater or transmitter relayCP, assumed to be of the two-position biased polar type, which whenenergized with its oper- 5 ating polarity shifts its contact finger 4 tothe left to connect the track battery 5 across the track rails in serieswith a limiting resistance 6 in ac icordance with the usual practice;when de- =eriergized movesits contact finger 4 to" the right 10 to thebiased position to connect an oif-code track relay OCTR. across thetrack rails with or without -a-seriesmesistance in accordance with ther'characteristics of the track circuit. This track relay is also of thetwo-position biased polar type,

right'fromdtsibiased position, energizes a front contact repeater'relayFTP, and when moved to wthe left tot-hetzbiased position energizes aback contact repeater relay BTP, provided the front '20 "contact"repeater relay FTP is then energized to :close :contact.36.-Ifboth-.ofithese; repeaterrrelays are. energized to. close contacts.31; and! therelay OCH is energized. .This'.ismerelyfrepresentative ortypical of .one type oL-meansfondetectini;

. that .theroff-codetrack relay QCTRisdnterrdittently operated .by theinverse codepulses. (Its can be readilytseen thatifrthistrackrelaywisvsteadily energized or deenergized,one :ortheotherqof-dtsrepeater. relays is deenergized,.-and--.the relayqficl'i 130 accordinglyreleased.

The significant feature-characteristic .'of;th.is invention is that theenergizin circuit riortthe transmittenrelay includes-a backcontactmmtthe off-code track relay .OCTR, .the, circuit being readilytraced from through=the-rcontact:8 of relay OCTR .closed "withithearmatureeof relay in its biased wor deenergized rposltion, through the:coding contacts-6mm or C1 5,.front or back contact 9 of relay .X,astthemasemaylbe,

-. other words, the off-code track rela iacfliRimust assume .its'deenergized conditioneaftereachcdflfcode pulse before thetransmitterwrelayneP'zoan be energized .to;. give: the .nextzoni periodztortthe transmitted-code.

The decoding apparatusiand'rmeansnfors-transmitting the ofi-code .5pulses ;at the otheruendmf the track-sectionmay-.beof any suitablearrangement in accordance=with=-regular;praotice. .rAsiil- '50lustrated, an impulse rrelay. IMP-:of .the two-potsition biased; polarvtype .-is-arrangedzto act, .when deenergized :and. its. contacttfin er etnriscrnovedtto the left, to connect the :code .followingzztrackcrelay.CFTR. across the track :railseaizrdwzhemen- -.ergized to connect [thebatterycl lz-foriathe ofiecdde pulse across the; track .rails. 1ilheecontactzflnger l2 of the code following tra'clrrrelaycrfmienergizes:alternately the two -;halves .:of ithe primary of .a idecodingtransformer -l 3;havingJa'. mldltap in' accordance '-with1the"1usual:practice, 1 and one .secondary of -'this transformer is :connectedthroughrrectifying: contacts I 4 on= this itraclc-relay .CF'IR toarrelay I I-Ltwhich is? energizedsdzlong .as the codefollowingtrackrrelayiscoded regardless ofiits;code'rate. Anothersecondary oftthe decoding transformeris arra'nged to energizer a circuitincluding La condenser 2 9 and tuned 'for the 'higher code'rateof I for-impulsi'ng the primary of the transformer 51 I 6, the i-secoridarymfwhich is connected "through a 'full wave rectifier H to a relay lD-forindicating that a code ofthe 180 rate is 'being transmitted. "The relaysflit-I and ID are assumed to control the'=green,' yellow,

vand-fred' indications of "the signal'l ins'the"usual manner.

The decoding transformer I3 is also provided with an impulse winding I8for energizing the impulse relay IMP when the code following track relayCFTR is deenergized at the end of an on period of the driven code. Thisimpulse winding I8 isso wound and connected to the winding of the relayIMP that the momentary voltage induced therein when the relay CFTR dropscauses current to flow through relay IMP in a direction to move itscontact finger I from the biased position, while the voltage induced inthe impulse winding when the relay CFTR picks up merely holds the relayIMP in its biased position to the the code oscillator CI8Il, forexample, closes its contacts for the on period, the transmitter relay CPis energized to connect the battery 5 across .the track rails and pickup the code following track relay CF'IR. This reverses the energizationof the primary of the decoding transformer I3 to provide energizingpulses for the relays [H and ID. When the code oscillator opens its con-,tact and relayCP drops, the battery 5 is dis-- connected and. theoil-code track relay QCTR is connected across the track rails; and whenthe code following track relay CF'IR releases, the impulse relay IMP ismomentarily energized in a direction to connect the battery II to therails to create an oil-code pulse to energize the relay OCTR, thisenergization being only momentary -so that relay OCTR. assumes itsdeenergized biased condition before or at the time the code oscillatoragain closes its contacts for the next on period.

Assume now that there is an extraneous or stray potential across thetrack rails due to ground currents or leakage from some source,

and that this stray potential is of the proper polarity and ofsuificient intensity to energizethe off-code track relay OCTR. Underthese conditions, when the transmitter relay CP is deenergized and therelay OCTR, is connected across the track rails, it is effectivelyenergized by this stray potential and is maintained efiectively energized, so that the circuit for energizing the transmitter relay CP isbroken at the contact finger 8 of the track relay OCTR, therebypreventing any further operation of the transmitter relay. This not onlyprevents the intermittent disconnection and connection of the off-codetrack relay OCTR to the track rails by the transmitter relay vCP and thepossibility of it being improperly operated by the stray potential, but

also stops the transmission of the drivencode .to put to stop signal I,thusv calling attention to the fact that abnormal conditions exist. Suchcessation of code transmission occurs as soon as the stray potentialappears across the track rails, regardless of whether or not off-codepulses then exist, and the apparatus is likewise automatically restoredto normal operation of the track circuit as soon as the stray potentialdisappears. Also, the protective feature is effective regardless of thepoint of application of the stray potential along the track rails, theresistance of the ballast or the bonding, or the relative intensity ofstray potential as compared with the voltage oi.v the track battery, thedetermining factor for cessation of code transmission being simply thatthere is a stray current present capable of energizing the oil-codetrack relay and causing false self-coding operation.

Since the transmitter relay CP in this embodiment of the inventionillustrated in Fig. 1 cannot be energized with the oscillator contactsclosed, unless and until the ofi-code track relay OCTR has released, itcan be readily appreciated that prolongation of the off-code pulse tendsto artificially prolong the off periods or the driven code and shortenthe on periods. It is ordinarily desirable, however, for satisfactoryand efficient decoding operation, particularly for the higher coderates, to have the code following track relay or its repeater operateits contacts at uniform intervals; and it is contemplated thatquick-acting relays will be used and the system otherwise so organizedthat the ofi-code pulse will fit in the normal off periods of the drivencode, with a reasonable margin to take care of variations in ballastresistance and battery voltage. In this connection, various expedientsmay be employed to control the duration of the off-code pulse, and alsoprolong artificially the controlling effect produced by a very shortofi-code pulse.

The specific embodiment of the invention has been illustrated anddescribed in connection with a single coded track circuit adapted foruse in various types of railway signalling systems in which it isdesirable to provide a safe inverse or off code in addition to a drivencode. As already indicated, the olT-code protective check afforded bythis invention is particularly useful where the off code is relied uponfor safety, and is cut, off prior to the entrance of a train into thecorresponding track section; and as a typical or illustrative example ofsuch an application of this invention, there is illustrated in Fig. 2 ina simplified and diagrammatic manner one organization of circuitsutilizing the principles of this invention in connection with approachlocking control.

In the arrangement illustrated in Fig. 2, it is assumed that a homesignal I of an interlocking or centralized traffic control systemgoverns train movement over a power-operated switch or switchesindicated conventionally at 25, 'and that the distant signal 2 and otherautomatic block signals (not shown) in the rear are controlled by codedtrack circuits, an inverse or off code being employed to provideapproach locking control for the switch. The particular organization ofap paratus and circuits governing the power operation of the switch orswitches and the indications of the home signal I by the supervisorymanual control of an operator in accordance with recognized practice, isnot material to the present invention; and to facilitate anunderstanding of the principles and mode of operation of such a system,a simplified arrangement of parts and cir cuits has been illustrated.The home signal I is manually controlled by movement of a signal leverGL to energize or deenergize a signal control relay GZ to close acontact 26, having an operative connection to the signal controlcircuits for signal I, so that this signal may be cleared, subject tosuch track circuit control as is customary, to give a high speed orrestricted speed :;-on :the; drawings; *and .this arelay 'ALS releases:its;armature.tomopen at: its contact 28 the :circuit 2 f orthe .switchcontrol iWZeandn prevent any-mpervationlrof :theuswitch rmachine. hile:sthe home .csignal I ..is atvstop rand back contact :30-1of :relay Gz-isxuclosed, :the:relay is maintained venerfigizedstzby azstick'circuitthroughiits front con- :tact 31 readily itrace'd on the drawings,inispite .cof Jthe :deenergization of the approach .relay Z- OCI-I; andopening of. itstfront contact-s3 Zrbyan tapproaching :train,.. ;so cthatthe iswitch may :loe coperated. .iI f :the "home :signal .al, havingonce izbeen cleared; :is manually: puttoi'stoplbwthe oper---atornpreparatory tochanging the route, :and if there is no'trainwithinthe :approach locking llimitssothat' relays 2-0CH is energized to: closeits lfront contact 32, the approach locking relay :may be energized: byits pick-up 'circuitand "the switch operated. It-however, ":there is atrainwpresent within the approach locking limits and 'relay 2-OCH isdeenerg-ized opening *contact 32, the approach locking relay ALS is notre-energized 1 by putting the signal I to stop, =and the switch islocked and cannot be operated --in front of-the approaching train. Inaccordance with the 'usualpractice, suitable provision (not 1illustrated) may 'be made to permit the emergency release of theapproachlocking by-a time element device orequivalent, so that aftertheapproaching train has' had time to stop inobe'dience to the restrictivesignal indications, the switch may be unlocked and operated.

Considering now the-use'of the inverse or off -code'for control -inapproach lo'cking, the limits for the approach locking control'usuallyinclude -a track circuit-section inthereanofthe'distant sig'nal=2, thenormal direction of tra'flicbeing fromlftto right. Instead of using'aline circuit to "deenergizean approachrelay adjacent the -switch'wheneither one orboth of the track sections T2 or T3 is occupied, it iscontemplated that an inverse oroff code in these track sections willbeemployed to indicate by the deenergization of the off-code responsiverelay 2OCH the presence of'an approaching train in either of these tracksections. "The track section T3'may extend through "the block' tothenextsignal in therearof signal'2, or mayinclude only a portion of thisblock; but in either case, it is provided "withacode following relay andan impulse "relay (not showni'at its entrance end in'the same way'asa't' the entrance end of the tracksectionTZ. 'Each of these tracksections isprovided' with an off code track relay' OCTR arranged tomaintain a slow-release decoding relay OCH energized so long as thistrack relay is coding. In the particular arrangement illustrated in thisFig. 2 when the contact finger 34 of the relay'OCTR is in the biaseddeenergized position, the primary of a transformer 35 is energized; andwhen this contact finger '34 assumes the operated, position, -asecondaryofthis' transformer 35 is connected release and openxcontactfidwhich inturnzstqps .-thetransmission: 'of iofi-colde pulsesi the-1mm]:section T2,;therebysreleasingfl Ocl-ii and maniwfesting the; presence;of: aneapproaching 'ztrainzin the approach locking limits.

. ti -Assume,'however,;that there should'r'bemvstray :potential .on:theutrackrails :ofisection' 'flhcapable of energizing the track relay2-OCTR, iti'oan'cbe seen ,that, withoutiithe check :or eprotectiveiteal):;ture:provided:by contactwfi'l V of this: inventiomzthis track relay;2.OCTR might lee-:filselysselfwode'd rabythis: stray currentand'maintain' relawZ-eOOH aenergized; in spite oil the 1 factxthat-zthesofi-oade pulsesihad: been 'cutcoff :by rthe presence :of zthe '25attain in'ithefltrack section 7123 openingtcontacttli.

'jthis JCODHEGtiOIl, it ;is intended i:that sthe spolaritycofi thetrackrbatterieseandathetopenating :polarity .of r the tra'ckrrelayswvi'll bGESOliSBlBCtdZBS to afford thesprotection against'brokeneflownin- -suiated rail iointszcharacteristicpfithencodeid trackcircuit;.=an'd with one combination:dfriaolaritiesaa stray -potentialucapable Lof l effectively aenergizing .xthecofi-zcoclectrack'relayofi'l Rzwillualso ztendlto energize zthe L Lcode f'following "track'srelay CFTR,

135 :so'that':thexpresence 10f aa' stray. currentmapable of "causingunsafe-falseiselfecocling of ithe :ofi-mo'de 'traclcrelay willfalso tendto -prevent itherreception 1 :of the driven 'code' by locking 'up theicodetollowing track relay 'GFIRgthereby putting 'the dissupplementalprotection against :improper operation by stray currents, since anapproaching train would receive a stop indication at 'signal ltif a'stray current exists, and hence -would'zbe proceeding-at a restricte'dspeed making itpermisfiifiie -to release the approach-locking. "Suchprotection is marginaL -howevenbecause with :low ballast resistanceorpoor'bonding; a; stray 'potential' near the-' exit end of the track'section' -T2 *maybe of suil'icient intensity to operate the oil-codetrack relay without locking up the code following track relay 2 cm. 'Itisnoted that -the=relays 26? and 30? are shown as 'neutral'type relaysin this IEig.'=2,"ib\:lt they may be polar relaysasin-Figil.

There are a n-umber'of'modificationsarrd-adaptations that may be made*in' the particular arrangement'and construction ofthe partsand-cirvcuits to carryout the principlesof-this'invention; and anumbero'f'modifications are i'llustrated in Figs.3 and 4.

Generally speaking; it-isdesirable'to'use'a mini- -'-mum' number ofcontact fingers on-'a"track' relay 5 respondingtodriven-orofi-codepulses 'forsatisfactory and efficientoperation,iparticuIaMywith long track circuitsaridypoorballast? and' atthe same time, it is expedient to employar decdding transformer forcontrolling "the"energization of In the "modification illustrated in"Fig. -3,*the contact finger of the oficode trackrelayUC'IR in thedeenergizedbiased position supplies-current to, the energizingcircuitfor the transmitterrreiay 5 "CP in accordancewiththe'jprinciple'ofprotection characteristieof this invention. Thissame contact finger 40 energizes and deenergizes the primary of atransformer 4| as the track relay is intermittently operated; and thesecondary of this transformer, in which impulse voltages are thus,induced, first in one direction and then in the other, suppliesuni-directional energizing current for the code-responsive relay OCHthrough a full-wave rectifier 42 of the usual type and construction.

Fig. 4 illustrates another modification in which the off-code trackrelay OCTR operates a repeater OCTP which controls the decodingtransformer and rectifying contacts for governing the ,relay transmittedin the opposite direction comprising,

, an oiT-code track relay responsiveto the off-code pulses, atransmitter relay intermittently ener-' gized to transmit a driven codeand to connect 1. In a coded track circuit for railroads comprising, asource of current and an off-code track relay associated with theexitend of a track circuit section, driven code transmitting means operatingto connect said source of current across the track rails intermittentlyto provide driven code pulses of 'a selected code rate separated by oilintervals and to connect said off-code track rela across the track railsduring each off interval, means including a code following track relayassociated with the other end of the track circuit section fortransmitting a momentary cit-code pulse during each off interval of thedriven code and thereby energize said off-code track relay,

code responsive means including a slow release relay maintainedenergized in response to the in-.

termittent operation of said track relay by offcode pulses but becomingdeenergized if said oficode track relay ceases operation, and means forcausing said driven code transmitting means to cease, operation if saidoff-code track relay is energized by a sustained potential across thetrack rails and hence fails to release near the end of an off intervalbetween driven code pulses whereby said driven code transmitting meanscannot cause false operation of said off-code track relay when asustained potentialis present across the track rails.

2. In a track circuit of the coded type for railway signaling systemsoperable to transmit an inverse code in onedirection during theoffintervals of a driven code transmitted in the opposite directioncomprising, an off-code track relay, a transmitter operatingintermittently to transmit driven code pulses and to connect said trackrelay across the track rails during each off interval between the drivencode pulses, and means for preventing said intermittent operation ofsaid transmitter while said off-code track relay is energized, wherebytransmission of both the driven code and the off code is automaticallystopped whenever said off-code track relay is maintained energized by astray potential .across the track rails Q 3, In a coded track circuitfor railroads .oper-.

able to transmit offcode current pulses in one direction during each offinterval of a driven code said off-code track relay across the trackrails during each off period of the driven code, and an energizingcircuit for said transmitter relay including a back contact of saidtrack relay, whereby operation of said transmitter relay isautomatically stopped if said track relay is energized by a straypotential across the track rails.

4. In a coded track circuit of the character described comprising, aself-propelled code transmitter, means operated by said transmitter foralternately connecting a source of current and a two-position biasedpolar oif-code track relay across the track rails at one end of a tracksec, tion, and means for preventing said alternate operation of saidtransmitter while said track relay is energized, whereby the presence ofa stray potential on the track rails of a polarit and intensitysufficient to eifectively energize said track relay automatically stopscoding operation of said track circuit. a

5. In a coded track circuit system of traific control comprising, asignal at an entrance end of av track section, a transmitter foralternately connecting a source of current and an off-code trackrelayacross the track rails at the exit end of said section at anyselected one ofa plurality of code rates, means for applying across thetrack rails at the entrance end of said section an oiT-code currentpulse during each of the off periods of the driven code for energizingsaid off-code track relay, decoding means associated with said signalfor governing its indications in accordance with the rate of driven codetransmitted, and means for automatically stopping operation of saidtransmitter and causing'said signal to indicate stop whenever saidoff-code track relay is mai ntained energized by a sustained current onthe track rails.

.6. In a system of traflic control for railroads employing a pluralityof coded track circuits each operable to transmit an inverse code in onedirection during the off intervals of a driven code in the oppositedirection, said system comprising, means responsive to the presence of atrain in one section for stopping the transmission of the inverse codein an adjoining section in advance, decoding means for detecting theinverse code in said adjoining section and including an off-code trackrelay connected across the track rails during the off periods of thetransmitted code,-said de-coding means being responsive onlyto thecontinued intermittent operation of said track relay,

and means for automatically rendering said decoding means ineffectivewhenever said off-code track relay is energized by a sustained potentialon the track rails.

7. In a system of traific control for railroads comprising, a pluralityof adjoining track circuit sections each equipped with -a coded trackcircuit operable to transmit an inverse code pulse in one directionduring each ofi interval between driven code pulses of a selectedcoderate transmitted" code track relay is operated intermittently, means forautomatically preventing transmission of driven code pulsesin a" tracksection Whileth'e inversemode track"relarior that track section is"energized, an'dzmeans iorstoppingthetransmis sionpfinverse-code"pulsesin a track'circuitsec tion wherrthe-codewesponsive' means of an ad"-joining; section" is deenergized.

8; lira system of approach lockingfor' poweroperatedtrack'switch'escomprising, a homesigna'l anda*di'stant'sign"a;l governintrain-movement towarda' switch; a-section of track between said' signalsequippedwith' a coded track circuit operabl'toapply 'a momentary offcode' pulse across tii'e'trackrails'at the-entranceend of 'saidsectionduringwach off interval between the pulses ofa." driven codeofaselectedcoderate'applied to'th'e' exit'endofsaiwtrack section" for governing theindications of said: distant signal, an off-code track relayy means-forintermittently connecting said oif-code'track relay-across the'track'railsat the-exit end of said track section during the offinterval of thedriven code; means responsive "totlie-presence-of' a-train' in' a 'track'sectiorr in the" rear' of the distant'signal for stoppingj'th'etransoperatewtrackswitchesia=homesignal governing tramc overttiiepoweroperated track-='switcli: and? at least two approachtraclesectionsleadingeto said' switch'; said traelseetiomnearestsaid switch havingadistant si'gnai governing; the entrance of trafii'c thereto;- codedtrack circuit apparatus associated with each of said track' sections forproviding 'a driven *code appliedto the rails at- 4 the-'exit end' ofthe-section-and"'also providiirg forthe transmission of an inverse codeby applying momentary pulses=acrosstlierail=attheentrance end of 'the*section -between *the impulses-or ent driven" code; said apparatusproviding-= tiiai'fltiie driven 'code oisaid' "near section' hasz'ara-te' in a-c cordance witli' tlie'indication' of said liome signal andproviding tliat"-'th'e'-d;riven code i in '-.said dist-- tant sectionhas a rateimaccordance witti'ti'aflic conditionsin said nearseetion;andsaid apparatus also providi'ng th'atthe=-inverse= code in said' neawmission of off-code pulses in said adjoining ti'aack' section; anapproach lockingistickrelay acting whendeenergized'topreventoperationofthe' track switch; an energizing" circuit for'saidap= proaclilocking stickrelay; code'responsive meansmaintained"energized" oniy by theintermittent operation of said off cod'e'track'relay for control ling'said' energizing'circuit, and meansforrerrd'eringsaidcoderesponsive means'ineffective if saidofflcode'trackrelay is'maintained" energized" bya"'sustained potential across-thetrack" rail'gofv power-actuated" codingcontacts; an" energizingcircuitfor" said transmitter relayincluding, said" coding: contacts and a: backcontact of said off co'detrack'relay "closed when that'relay'is'd'eener: gized, a decodingtransformer'liaving th'e'energicationofits'primarycontrolledby'operati'onoi said oft"--codt t'ra'ck relay, anda cod'Tespon'sive' relay, "connected to ttiesecon'da'ry of said" transformer and-temporarily" energized riy "the" impulse voltages'pro'ducedbysaicl'transformer'as'said ofl code 'tra'ck'reiay'is intermittently;operated:

10. In a coded track circuit fbrrailroadscomprising; a source" ofcurrentand a track: relay associatedwith one" end of a; track'section",a

transmitterwelzyactitxgwhen energized and deenergized"to connect"alternatelrsaid source of current,- and said track" relay" respectivelyacross' tlietrack' rails" of said track section; decoding meansmaintained energizedonly by. the repeated operations of said'track"re1ay,"power' operated codingcontactsintermittently"opened and closed at'mselect'ewcodiate'; andan energizing circuit for'said'transmitt'errelay, inciudingin"seriessaidf co;i irrg"'contacts*and"a back-contactofsaid track re ay:

11: Ina" system ofapproach looking for power section shall Ioe dependentupon the presence' of" arr inverse co'd ir'r said dis'tant section;approach code responsive means adjacent said tracl switchincluding-artracl relay responsive to-tli'e dnverse' code= pulses andaHome relay maintained ener giZed' only upon the continued reception ofan inverse code by said track relay, saidmeans -aiso acting to preventtransmission oI- said driven'code if i said track" relayremainssteadily" energized, manually -'oper able means for governing:the indi-fcations'pfsaidiiome signal, an approacli lockmgi stick'relaypreventing power operation o'fi said tracle' s'vitcir-when itisdeenerg-iz'ed={ means effec tive'* to denergize said approachlockingstick relaywl'iensaid-Home-signai'is 'controiled toiindi T cateclear; and apick uprcircuitffir'saidapproacti looking "stick ='relay"closed onlywlien said--"h'omle reiay is energized; whereby the presenceof a train in eitiier'saidinear or 'said distimt section is efl'ctive'to govern-themicmup'ofsaid approach" 1ocking="stick reiaav'.

12? In a coded track circuitfor=railioads=of tiie=- type liavingcodeipul'ses transmitted in bothditec trons; a: transmittingandrecelving means atpne end of? the ti'aclc circuit comprising a sourceof" current; a track relay; atransmitter relay eflec tive wliemenergized and denergized to alter natelir connect said" source'of'current and i said track relay across :said' track'circuit'; coding0011+ tacts 'opened'=and* closed witha predetermined periodicity, anenergizing circuit for said t'rans mitten-relayincluding said' codingcontacts andv a" contactclosed only 'wh'ensaid track relay is-'deenergized; anddecodingmeanscontrolled by"tlie= intermittentoperationof said*traci relay; where by said transmitter relay isrendered ineffectivev to transmit? acod- -pulse whilesaidtrackrelay isenergizedf 13. In a: coded track circuitfdr railroads-, the combinationwiti'i a" track circuit section, driven code' transmittingj means 'for-'appl$ring across' the track rails at* the exit endofsaid sectiondriven code; puls'esatany selected one ofa" plurality "of diflrentcode'rates: saidfdrivenccodpulSesrbuging-z separatedfbycfl periodsj' vmeansincluding a: code: v

followingttra'clrelay connected across tli-track raiis at the oppositeend'i'oi "saidsecticm ion-dc- 'co-dir'rgsaid driven code;meanswesponsive' tb'theoperation of said? code followingtrack relay forat times applying at said* opposite' end" off'said" section-an oifcode'current'pulse during a portion of each of the off neriodsoi .the.driven codes: an off code'ftrackrelay at saidexit end-of said sec.-

tion intermittently energized by* said offcodia pulses; decodingmeans-including a slow acting relay, circuit means for normallymaintaining said decoding means effective to energize said slow-actingrelay during continued intermittent normal operation of said off-codetrack relay by said oiT-code pulses, and contact means acting on saiddriven code transmitting means to render said means inefiective wheneversaid off-code track relay is subjected to a steady stray potentialacross the track rails, whereby said code following track relay ceasesoperation and thereby prevents the application of off-code currentpulses during the presence of a steady stray potential across the trackrails. v

14. In acoded track circuit for railroads, the combination with a tracksection, a source of current, an ofi-code track relay, code transmittingmeans for intermittently connecting said source of current and saidtrack relay alternately across the track rails of said track section atone end to provide driven code pulses separated by off intervals, meansassociated with the other end of said track section for applying to thetrack rails oficode pulses of limited duration during the oil intervalsbetween said driven code pulses, code responsive means energized only inresponse to the intermittent operation of said track'relay by oficodepulses, and means for preventing operation of said code transmittingmeans and the disconnection of said track relay from the railsof saidtrack section by said code transmitting means if said track relay isenergized by a sustained potential across the track rails, whereby thepresence of a sustained potential across the track rails results in thedeenergization of said code responsive means NEIL D. PRESTON.

